Fourier-transform infrared spectrometry is an established and very powerful tool for the spectral analysis of light emitted and transmitted by gases, liquids and solids. In its most common form, a Fourier transform spectrometer includes two mirrors located at a right angle to each other and oriented perpendicularly. A beamsplitter is positioned at the intersection of the reflection of the mirrors and oriented at a 45° angle relative to the mirrors. Radiation, such as light, is directed at the beamsplitter such that the beamsplitter divides the beam into two parts. The parts of the beams are reflected off corresponding mirrors and recombined. The recombined parts of the beam are sensed by a detector at the output of the spectrometer. It can be appreciated that the signal recorded at the output is dependent upon the wavelength of the light and the optical path difference between the beamsplitter and each of the two mirrors. By varying the position of one of the mirrors along a predetermined axis, an interference pattern is generated.
In heterodyne spectroscopy, the different colors of light that are naturally encoded by their unique wave frequencies are exploited. These frequencies are generally too high to be detected directly. As such, frequency down-conversion can be achieved by mixing a light wave with narrow-bandwidth light of a known, but slightly different frequency. Consequently, the envelope of light will oscillate at a beat frequency, i.e., the difference between the frequencies of the two light waves. The beat frequency can be tailored to reside in the radio frequency and lower frequency bands. It becomes a simple task to measure the amplitude of the beat frequency using standard detectors and standard data acquisition devices like oscilloscopes. However, it can be appreciated that researchers often need to measure more complicated spectra, encompassing thousands of data points, rather than just two. Consequently, in order to expand the basic heterodyne spectroscopy approach to full spectral measurements, one would need a large number of lasers with known frequencies. Hence, this approach can be unduly burdensome.
In order to overcome the limitations of the basic heterodyne spectroscopy approach, mode-locked femtosecond lasers have been used. As is known, the spectrum of a mode-locked femtosecond laser is defined by a comb of equidistant frequencies called modes. By overlapping the output of two lasers that generate slightly different combs, one can generate a multiplicity of down-converted frequency pairs. More specifically, the difference in the spacings between the combs is kept so small that the beat frequency of the two modes is significantly smaller than the mode spacing of a single comb. The combined beams from the two lasers will exhibit beats at multiples of the difference in the spacings between the combs. If the spectrum of the light is inferred from the Fourier transform of the signal's time trace, the spectrum will exhibit a low-frequency comb with the above-described spacing due to the optical beating. As a result, any frequency pairs would be attenuated and show up in the beat spectrum. By superimposing the two combs, a beat frequency in a low-frequency region is assigned to an optical frequency.
While functional for its intended purpose, it can be appreciated that the use of mode-locked femtosecond lasers is cost prohibitive for most applications. Individual femtosecond lasers can cost several hundred thousands of dollars. As a result, it is highly desirable to provide a simple and inexpensive heterodyne frequency-comb spectroscopy system that allows for full spectral measurements. In addition, the use of mode-locked femtosecond lasers generally requires the use of moving parts, which adds to the cost and complexity of this approach.
Therefore, it is a primary object and feature of the present invention to provide a heterodyne spectroscopy system that allows for full spectral measurements.
It is a further object and feature of the present invention to provide a heterodyne frequency-comb spectroscopy system that is inexpensive and technically simple.
It is a still further and object of the present invention to provide heterodyne frequency-comb spectroscopy system that utilizes continuous-wave, frequency-comb generators and is free of moving parts.
In accordance with the present invention, an apparatus is provided for conducting heterodyne frequency-comb spectroscopy. The apparatus includes a first frequency-comb generator for generating a first continuous wave laser beam. The first beam defines a spectrum of light that includes a plurality of optical frequencies spaced by a first frequency. A second frequency-comb generator generates a second continuous wave laser beam. The second beam defines a spectrum of light that includes a plurality of optical frequencies spaced by a second frequency. A beam combiner is operatively connected to the first and second frequency-comb generator. The beam combiner combines the first and second beams and provides the same as a combined beam. The combined beam has a plurality of beat frequencies dependent upon the optical frequencies of the spectrums of light defined by the first and second beams.
The first frequency-comb generator includes a filter having an input and an output. The filter controls the spacing of the plurality of optical frequencies of the first beam. The filter includes an etalon and the first frequency-comb generator includes a controller operatively connected to the filter. The controller controls the temperature of the etalon. The spacing of the plurality of optical frequencies of the first beam is dependent on the temperature of the etalon.
The first frequency-comb generator includes an optical amplifier for generating an initial laser beam having predetermined optical power and an output coupler. The output coupler generates the first beam from a first output portion of the optical power and directs a feedback portion of the optical power to the input of the filter. The first output portion of the optical power is generally equal to 2 percent of the optical power.
The first frequency-comb generator may also include an isolator operatively connected to the linear optical amplifier and to the optical coupler. The isolator insures the optical power propagates in a first direction. An optical attenuator has an input operatively connected to the output of the filter and an output. A polarization cavity has an input operatively connected to the output of the optical attenuator and an output operatively connected to the input of the optical actuator. The optical attenuator controls the optical power of the initial laser beam. The polarization cavity maintains the linear polarization of the feedback portion of the optical power.
A spectrum analyzer may be operatively connected to the beam combiner. The spectrum analyzer records the combined beam. An enclosure is provided for receiving the first and second frequency-comb generators therein.
In accordance with a further aspect of the present invention, an apparatus is provided for conducting heterodyne frequency-comb spectroscopy. The apparatus includes first and second frequency-comb generators. Each frequency-comb generator includes a laser cavity and an output coupler. The laser cavity propagates optical power traveling thereon. The output coupler optically communicates with the laser cavity for receiving the optical power. The output coupler generates a continuous wave laser beam from a first output portion of the optical power. The continuous waver laser beam defines a spectrum of light that includes a plurality of optical frequencies spaced by a frequency. A beam combiner is operatively connected to the first and second frequency-comb generators. The beam combiner combines the first and second continuous wave laser beams generated by the first and second frequency-comb generators and provides the same as a combined beam. The combined beam has a plurality of beat frequencies dependent upon the optical frequencies of the spectrums of light defined by the first and second continuous wave laser beams.
Each frequency-comb generator includes a filter optically communicating with the laser cavity. The filter includes an etalon and each frequency-comb generator includes a controller operatively connected to the filter. The controller controls the temperature of the etalon. The spacing of the plurality of optical frequencies of the spectrum of light is dependent on the temperature of the etalon. The first output portion of the optical power is generally equal to 2 percent of the optical power. A spectrum analyzer may be operatively connected to the beam combiner. The spectrum analyzer records the combined beam.
In accordance with a still further aspect of the present invention, a method is provided for conducting heterodyne frequency-comb spectroscopy. The method comprises the steps of generating at least one continuous wave comb defined by a plurality of optical frequencies. The optical frequencies of the at least one continuous wave comb are down-converted and an optical spectrum for the at least one continuous wave comb is determined from the down-converted optical frequencies.
The step of generating the at least one continuous wave comb includes the steps of generating a first continuous wave laser beam having a plurality of optical frequencies spaced by a first frequency that define a first continuous wave comb and generating a second continuous wave laser beam having a plurality of optical frequencies spaced by a second frequency that define a second continuous wave comb. The step of down-converting the optical frequencies includes the step of superimposing the first and second continuous wave combs to determine a beat spectrum. The step of generating the first continuous wave laser beam includes the step of generating optical power having a spectrum. At least a portion of the optical power is filtered so that the spectrum has the plurality of optical frequencies spaced by the first frequency. The first continuous wave laser beam is generated in response to the spectrum. The step of filtering at least a portion of the optical power includes the additional step of passing the portion of the optical power through an etalon.
In accordance with a still further aspect of the present invention, a method is provided for conducting heterodyne frequency-comb spectroscopy. The method includes the step of generating at least one continuous wave comb defined by a plurality of optical frequencies. The plurality of optical frequencies is designated to generate modulations at lower beating frequencies and the lower beating frequencies are designated to observe an optical spectrum.
The method may include the additional step of exposing the at least one continuous wave comb to a predetermined stimulus. The exposing step may include the step of passing the at least one continuous wave comb through a sample. The at least one continuous wave comb may be generated by generating a first continuous wave laser beam having a plurality of optical frequencies spaced by a first frequency that define a first continuous wave comb and generating a second continuous wave laser beam having a plurality of optical frequencies spaced by a second frequency that define a second continuous wave comb. In addition, the step of designating the plurality of optical frequencies may include the step of superimposing the first and second continuous wave combs to generate the lower beating frequencies.